Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue by product
Product, net	$ 2,712.4	$ 2,622.5	$ 2,639.7	$ 2,380.1	$ 2,502.9	$ 2,539.6	$ 2,466.0	$ 2,309.4	$ 10,354.7	$ 9,817.9	$ 9,188.5
U.S
Revenue by product
Product, net									7,017.1	7,050.4	6,545.8
Rest of world
Revenue by product
Product, net									3,337.6	2,767.5	2,642.7
TECFIDERA
Revenue by product
Product, net									4,214.0	3,968.1	3,638.4
TECFIDERA | U.S
Revenue by product
Product, net									3,294.0	3,169.4	2,908.2
TECFIDERA | Rest of world
Revenue by product
Product, net									920.0	798.7	730.2
AVONEX
Revenue by product
Product, net									2,151.5	2,313.5	2,630.2
AVONEX | U.S
Revenue by product
Product, net									1,593.6	1,675.3	1,790.2
AVONEX | Rest of world
Revenue by product
Product, net									557.9	638.2	840.0
PLEGRIDY
Revenue by product
Product, net									494.3	481.7	338.5
PLEGRIDY | U.S
Revenue by product
Product, net									295.5	305.0	227.1
PLEGRIDY | Rest of world
Revenue by product
Product, net									198.8	176.7	111.4
TYSABRI
Revenue by product
Product, net									1,973.1	1,963.8	1,886.1
TYSABRI | U.S
Revenue by product
Product, net									1,113.8	1,182.9	1,103.1
TYSABRI | Rest of world
Revenue by product
Product, net									859.3	780.9	783.0
FAMPYRA
Revenue by product
Product, net									91.6	84.9	89.7
FAMPYRA | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FAMPYRA | Rest of world
Revenue by product
Product, net									91.6	84.9	89.7
ZINBRYTA
Revenue by product
Product, net									52.7	7.8	0.0
ZINBRYTA | U.S
Revenue by product
Product, net									0.0	0.0	0.0
ZINBRYTA | Rest of world
Revenue by product
Product, net									52.7	7.8	0.0
SPINRAZA
Revenue by product
Product, net									883.7	4.6	0.0
SPINRAZA | U.S
Revenue by product
Product, net									657.0	4.6	0.0
SPINRAZA | Rest of world
Revenue by product
Product, net									226.7	0.0	0.0
ELOCTATE
Revenue by product
Product, net									48.4	513.2	319.7
ELOCTATE | U.S
Revenue by product
Product, net									42.2	445.2	308.3
ELOCTATE | Rest of world
Revenue by product
Product, net									6.2	68.0	11.4
ALPROLIX
Revenue by product
Product, net									26.0	333.7	234.5
ALPROLIX | U.S
Revenue by product
Product, net									21.0	268.0	208.9
ALPROLIX | Rest of world
Revenue by product
Product, net									5.0	65.7	25.6
FUMADERM
Revenue by product
Product, net									39.6	45.9	51.4
FUMADERM | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FUMADERM | Rest of world
Revenue by product
Product, net									39.6	45.9	51.4
BENEPALI
Revenue by product
Product, net									370.8	100.6	0.0
BENEPALI | U.S
Revenue by product
Product, net									0.0	0.0	0.0
BENEPALI | Rest of world
Revenue by product
Product, net									370.8	100.6	0.0
FLIXABI
Revenue by product
Product, net									9.0	0.1	0.0
FLIXABI | U.S
Revenue by product
Product, net									0.0	0.0	0.0
FLIXABI | Rest of world
Revenue by product
Product, net									$ 9.0	$ 0.1	$ 0.0